The invention relates generally to laser velocimetry and more specifically concerns an auto/cross-covariance processor for use with a computer in making rapid covariance computations from data obtained from a laser velocimeter.
The laser velocimeter is a non-intrusive flow field diagnostic technique which measures the velocity of micron sized particles embedded in the flow field. If these particles are small enough to faithfully follow the expected flow field dynamics, then the results from the laser velocimeter will be a true representation of the flow field characteristics. The measurement of velocity is made when a particle passes through the fringe pattern within the laser velocimeter sample volume yielding a real time sample of the velocity flow field. A large number of velocity measurements is obtained to minimize the statistical uncertainties in the calculations of the basic flow field statistics such as mean velocity and turbulence intensity. As with any sampling technique, the assumption that the flow is both stationary and ergodic is made. With the exception of the length of time for which the stationarity assumption is valid, the times at which the velocity samples are made and the total sample time is not important for the determination of the basic statistical parameters. This, however, is not the case for time dependent characterizations of the flow field such as correlation functions and turbulence power spectra.
Classic approaches to the determination of the correlation functions and turbulence power spectra utilize direct correlation techniques and/or Fourier transform techniques using uniformally sampled data. The equation for the classic approach to determine the auto correlation function is: ##EQU1## When uniform sampling is used, the value of .tau. is the constant interval between samples and the kth location in the correlation is located as k.tau.. In laser velocimeter applications, however, the sampling process is random in nature since the particles which are measured arrive in a Poisson random manner and thus not applicable for classic approaches. Hence, a modification of the direct correlation approach must be used in which the random interarrival times, (the time between successive velocity measurements), are used to determine the appropriate delay time interval into which the cross product is to be summed. However, when this modification is used a large amount of computer time is needed to make the necessary calculations.
It is therefore an object of this invention to provide apparatus for measuring correlation function from data obtained by a laser velocimeter.
Another object of this invention is to provide hardware for use with a computer for measuring correlation functions from random sampled data.
A further object of this invention is to provide hardware for use with a computer to increase the speed of computations of correlation functions from data obtained by a laser velocimeter.
Other objects and advantages of this invention will become apparent hereinafter in the specification and drawings.